Gravity map reveals how water once flowed on Mars

One of the most accurate maps ever made of Mars might look a bit deceiving at first, given that it is mostly a putrid shade of luminous green. But in fact Nasa'snew gravitational chart of the (mostly) red planet has revealed new insights about our closest neighbour, by examining tiny tugs and pulls felt by spacecraft in orbit around it.

The space agency monitored small changes in the orbits of three craft currently circling Mars -- Mars Global Surveyor, Mars Odyssey and the Mars Reconnaissance Orbiter -- for more than a decade to make the new map. Collecting signal data from these craft using the Deep Space Network array of antennas located around Earth, Nasa was able to detect tiny variations in their orbits and map what that implied about Mars' composition.

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For instance, Nasa explained, Mars' gravitational pull is slightly stronger over mountains than over valleys, and changes when close to craters and other prominent features. Nasa also had to correct for the impact of sunlight on orbiting craft's solar panels, and even the pull of the atmosphere across 16 years of study to build the maps.

Gallery: Gravity map reveals how water once flowed on Mars

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"Gravity maps allow us to see inside a planet, just as a doctor uses an X-ray to see inside a patient," explained Antonio Genova of the Massachusetts Institute of Technology, who led the study at Nasa's Goddard Space Flight Centre.

The map can spot anomalies just 100km across, and has helped pin down the thickness of the Martian crust to within 120km. "The new gravity map will be helpful for future Mars exploration, because better knowledge of the planet's gravity anomalies helps mission controllers insert spacecraft more precisely into orbit about Mars," said Genova.

The map has revealed new details about features on and below the surface of Mars. One such anomaly, an area of surprisingly low gravity between Acidalia Planitia and Tempe Terra, is thought to have been the site of buried channels that carried water and material from the southern highlands to the northern lowlands, when Mars was covered in relatively huge amounts of water billions of years ago. The new map appears to prove this anomaly is present, and that it runs in the right direction for a downhill flow -- though it may also have other causes, Nasa said, perhaps including a 'bending' of the surface due to the weight of volcanoes on the Tharsis region of the planet.

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The new map also adds weight to the theory that Mars' outer core is made up of liquid molten rock. The team were able to monitor how the pull of Mars' two moons, and the Sun, affected its gravity map, indicating liquid rock was pulled in their direction, affecting the orbits of Nasa spacecraft.

Nasa added that it was able to infer how much carbon dioxide was frozen out of the atmosphere onto the Martian ice caps from the study, and understand better how that mass -- which can comprise up to 16 percent of the entire Martian atmosphere -- moves between the poles as the seasons change.

Similar gravity maps have been made of Earth in order to better study climate change and our own atmosphere and Nasa is currently requesting more money to make more detailed maps of our planet for this purpose.